Halftone images are commonly used in printed material, and now additionally in computer displays, to represent continuous-tone images in systems where only two levels (usually black and white) can be represented at any point. The term halftone, or equivalently binary image, means an image formed by black dots of various sizes so as to give the effect of continuous tone when viewed at normal reading distance. This definition includes the digital case where each black or white spot is a fixed size in an array, such that the variable size dots are generated by turning several adjacent spots black; as well as the case where the spots are not grouped together, but alternated to reduce the visibility of the patterns.
A common method of producing a halftone image is the use of a screen consisting of a pattern which has the same fundamental frequency in two orthogonal directions. The halftone screen is combined with the pictorial information. In a photographic method this involves imaging the picture through the screen, thus multiplying the transmittances. In most electronic systems, the screen and pictorial information are added. It should also be noted that in a digital system both the screen information and the pictorial information are sampled functions.
When the screen is periodic, the fundamental period of the screen is substantially larger than the sample interval, giving many samples per period. For both photographic and digital methods the combined pictorial and screen information is next subjected to a threshold. This is accomplished by recording on a high-contrast film for photographic halftoning or by a numerical comparison for digital halftoning. In either case, the result of the threshold operation is to produce a binary image.
In the usual method, the threshold is a fixed value and adjustment of the screen pattern (including so-called bump and flash exposures in the photographic case) is used to adjust the effective grey scale of the halftone as desired. This process results in dots of varying size, shape, and location within their repetitive pattern. In the digital case, each sample of the halftone screen and corresponding sample of the pictorial information are combined and result in one bit, which is then printed either black or white at a given location. Since there are a number of samples within each cycle of the halftone screen, several adjacent bits normally combine to give the effect of a single halftone dot with size, shape, and location depending on the pattern of bits.
Most halftone methods do a good job of giving the proper illusion of grey scale for low-spatial-frequency information on the continuous-tone image and partial dots in the halftone allow representation of higher frequency detail when detail contrast is sufficient. When the fine detail is periodic, however, spurious low-frequency patterns occur.
One prior art method for converting a continuous-tone image to a halftone incorporates the capability for both suppression of spurious (aliasing) signals and edge enhancement. The basis of the method for suppression of spurious signals and edge enhancement is to adjust the threshold for each halftone cycle in a manner which guarantees that the resultant halftone image matches the average reflectance of the original image. U.S. Pat. No. 4,051,536 is an example. This approach preserves the characteristics of the original halftone process such as partial dots. In areas of uniform grey in the original, the adjustable threshold will remain constant.
In general, the pictorial information over the area corresponding to one halftone cycle in two dimensions is averaged or else a low-pass filtered value is used, giving only low spatial frequency information in either case for control of the threshold. If grey scale information is to be preserved, this average determines precisely what percentage of the area must be covered by the halftone dot. In a digital system, this is equivalent to the number of bits which must be black of the total number of bits. Starting with the complete area of the binary image over one halftone cycle either all black or all white, the threshold is set at the extreme value to generate this case. The threshold is next adjusted monotonically. Either the total number of bits or dot size is examined during the adjustment process. As soon as the correct dot size is reached, the threshold value is fixed and the binary image is generated for that cycle of the halftone.
One difficulty with the system as taught by U.S. Pat. No. 4,051,536 is that to enhance the edges or get better edge detail, the screen contrast is turned down. It is, therefore, necessary to decide what screen contrast to use since enhancing the edges increases noise. That is, a high contrast screen lowers the noise in the image but provides no additional edge enhancement. Whereas, a low contrast screen enhances the edges but increases noise.
A solution to this dilemma is taught in U.S. Pat. No. 4,633,327 issued on Dec. 30, 1986, and assigned to the same assignee as the present invention. In particular, maximum and minimum detectors are provided to detect maximum and minimum pixel grey levels to dynamically adjust screen amplitude. That is, the screen amplitude is controlled on a dot by dot basis to selectively enhance the original image. There is a higher screen amplitude when the input image contrast is low and a lower screen amplitude when the input image contrast is high. Thus, the partial dots more closely follow image detail at high image contrast, but without enhancing noise in uniform areas. A difficulty with this system, however, is often the observance of "Moire" patterns and noise in the image. The "Moire" patterns were more apparent at higher enhancement levels. Haloes and similar artifacts also occur at edges
It is, therefore, an object of the present invention to provide a new and improved screening technique and method of enhancing a digital halftone image. It is another object of the invention to provide for image enhancement and at the same time to limit noise generation and reduce remnant "Moire" patterns and haloes artifacts. Another object of the present invention is to separate the uniform grey components from the fluctuating grey components in the area of each halftone dot.
Further advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.